Re: [BUG] VFIO on POWER9 / QEMU fails to allocate 32-bit DMA

Timothy Pearson [off-list ref] writes:

----- Original Message -----

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From: "Ritesh Harjani" <ritesh.list@gmail.com>
To: "Timothy Pearson" <tpearson@raptorengineering.com>, "linuxppc-dev" <redacted>
Cc: "Shivaprasad G Bhat" <redacted>, "Gaurav Batra" <redacted>
Sent: Thursday, August 6, 2026 12:27:22 AM
Subject: Re: [BUG] VFIO on POWER9 / QEMU fails to allocate 32-bit DMA

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Hi Timothy,

Timothy Pearson [off-list ref] writes:

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On Linux 6.18, and potentially earlier Linux versions, attempting to pass
through any PCI device via VFIO that requires 32-bit DMA results in the
following error in the guest VM:

Can you confirm if you see the same issue on upstream kernel also or is
it only till v6.18 kernel?

I'll need to check, but I don't see why it would be limited to 6.18.  This used to work, but it's fairly obvious IBM hasn't been testing or fixing any of the VFIO or nested virtualization code for years now, especially on PowerNV.

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[   11.705413] cx23885 0000:00:02.0: Warning: IOMMU offset too big for device
mask
[   11.709245] cx23885 0000:00:02.0: mask: 0xffffffff, table offset:
0x800000000000000

The same devices work correctly on the host kernel.  It appears that somehow
32-bit DMA is not available within a pSeries guest on a PowerNV host.

By any chance did you do kexec in the guest kernel? Is this issue only
hitting with kexec or even with normal boot up of guest kernel is also
showing up the same issue?

Yes, we kexec().  This works on every other architecture, not sure why kexec() would break it?

In any case, from where I sit at this point it's IBM's responsibility as the silicon vendor to ensure basic functionality works -- I don't see these issues on amd64 or arm64, and frankly on this end we're tired of the constant lack of QA from IBM causing every single kernel upgrade to be a game of Russian Roulette.  IBM should be able to spin up a test machine easily enough to reproduce, this isn't complex -- any card that needs 32-bit DMA is affected including inexpensive ASmedia USB 3 controllers.  Pass it through to QEMU via VFIO and watch it fail, it happens every time.

I was trying to help by asking for basic help from you. If you report a
problem upstream than we do expect such basic help e.g. is it only seen
with kexec, logs etc, so that we can all save some time here.

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Could you please share the dmesg logs for both guest and host kernel?
That might give some more data about this issue.

I don't have access to the setup at this point since it was already replaced with something else to restore functionality.

I think this patch [1] might have fixed the issue that you reported and
hence wanted to check if you only see the issue with kexec or also with
reboots? And if you could have given it a try.

[1]: https://lore.kernel.org/all/20260803224029.60538-1-gbatra@linux.ibm.com/ (local)

But anyways - I have already asked others on that thread (who are expert
of this area) to confirm if this patch could also fix the error you
reported with 32-bit DMA devices.

-ritesh